If you go to their website they have an annoucement right there. Looks like you will get OBC (on board credit). Here is announcement:

As fuel prices continue to decline, Carnival Corporation has announced that it will suspend fuel supplements for its six North American brands, including Princess Cruises, for all voyages departing on or after December 17, 2008.

For these sailings, a refund of the fuel supplement will be made in the form of a shipboard credit for all bookings within the final payment period. All bookings outside of the final payment period will be adjusted to remove the fuel supplement, and passengers will be provided with a revised final payment amount.

The company reserves the right to re-instate the fuel supplement for all passengers at up to $9 per person per day if the NYMEX oil price exceeds $70 per barrel even if the fare has already been paid in full.